Monitoring retinal blood flow appears crucial to understand the pathophysiology of ocular diseases such as glaucoma, diabetic retinopathy, and age-related macular degeneration (AMD).
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| LDH setup |
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| Montage of LDH images |
Laser Doppler Holography (LDH) is a full-field imaging technique that measures blood flow, specifically in the retina and choroid, by recording the interference between light backscattered by moving blood cells and a reference beam.
The LDH technique allows parallelized imaging, higher acquisition speed, and offline numerical processing.
It also allows full-field imaging, which is not possible with OCT angiography (OCT-A).
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| LDH, OCG, OCT-A, OCT |
In LDH, the optical field serving as non-Doppler shifted light is a separate reference beam beating against the Doppler shifted optical field backscattered by the retina. This allows the power of the reference field impinging on the sensor to be altered and thus be able to work with very low exposure time.
Interferograms are recorded with a high throughput camera and wideband measurements of the beat frequency of digitally reconstructed holograms are performed.
The angiographic contrast is drawn from the Doppler spectral broadening of light backscattered by the retina and blood flow changes during cardiac cycles as revealed using a short-time Fourier transform analysis. Such short acquisition times in the realm of a single cardiac cycle are not possible with OCT-A.
The light source used in LDH is a 45 mW, single-mode, fiber diode laser (Newport SWL-7513-H-P) at wavelength λ = 785 nm. The retina is illuminated with a 1.5 mW constant exposure over 2.4 × 2.4 mm2 area.
The laser beam is focused in the front focal plane of the eye so that the light is collimated inside the eye and illuminates the retina on an extended area.
A Polarizing Beam Splitter (PBS) cube is used in the object arm to illuminate the eye and collect the light backscattered by the retina.
The object and reference waves are combined using a non-polarizing beam splitter cube and they interfere on the sensor plane.
Power Doppler images drawn from the Doppler power spectrum density qualitatively show blood flow in retinal vessels over 512 × 512 pixels covering 2.4 × 2.4 mm2 on the retina with a temporal resolution down to 1.6 ms.
CONCLUSION:
LDH is used for non-invasive, high-resolution imaging of blood flow in the retina and choroid, assisting in studying hemodynamics in healthy and diseased eyes. It offers coherent gain, allowing for high sensitivity, and enables digital aberration correction to improve image quality.
